1. Field of the Invention
This invention relates to a polyethylene composition useful in rotational molding. The rotationally molded articles therefrom are characterized as exhibiting enhanced impact strength, luster and mold-release characteristics.
2. Description of the Prior Art
Rotational molding is effected by supplying to a mold a powdered thermoplastic material and rotating the mold uniaxially or biaxially while externally heating the mold to fuse the thermoplastic powder and then cooling the mold to cause the molded article to solidify. Rotational molding has a feature such that it can result in hollow articles which are, as compared with those obtained by blow molding methods, complicated, large in size and uniform in wall thickness, and further the material loss is minor. However, rotational molding has the following defects and therefore, rotationally molded articles are limited in their applications.
(1) A powdered thermoplastic material is heated through the wall thickness of a mold to be thereby fused. Accordingly, a substantially long period of heating and a high heating temperature are necessary.
(2) In a step of rotational molding, a fused thermoplastic material flows and spreads to form a fused thermoplastic layer on the inner surface of a mold. Therefore, if the thermoplastic material used is poor in flow characteristics, bubbles are liable to enter the fused thermoplastic layer and the surface of the fused thermoplastic layer becomes roughened, and consequently, the resulting molded articles are not of an attractive appearance.
(3) If a thermoplastic material of enhanced flow characteristics, i.e., having a low molecular weight, is used, the resulting molded articles will be poor in impact strength and environmental stress cracking resistance (ESCR), and thus, in service life.
(4) A thermoplastic material is not subject to pressing during rotational molding. Therefore, the dimension of the resulting molded articles varies depending upon the time at which they are demolded from the mold in the solidifying stage. If the molded articles are demolded early, they shrink to an appreciable extent and thus are distorted.
(5) A molding apparatus is difficult to automatically operate. Both the opening and closing of the mold and demolding of the molded articles must be manually operated.
In order to obtain good molding characteristics to produce well formed rotationally molded articles, it is generally desired to use thermoplastic materials exhibiting a high fluidity, good release characteristics, a high impact strength and a low shrinkage. However, the higher the fluidity, generally the lower the impact strength. No ethylene polymers have hitherto been known to possess satisfactory fluidity and impact strength.
It has been proposed to use a high fluidity polyethylene having an organic peroxide crosslinking agent incorporated therein. For example, U.S. Pat. No. 4,029,729 teaches that a polyethylene composition comprised of an ethylene homopolymer or copolymer possessing a melt index of at least about 10 and having incorporated therein a crosslinking acetylenic diperoxy compound results in rotationally molded articles having a high impact strength and resistance to stress cracking. However, the use of an organic peroxide crosslinking agent has the following difficulties. First, as crosslinking advances, its adhesion to the mold increases, and hence, the resulting solidified molded article cannot be demolded from the mold without the use of release agents such as silicone and fluorocarbon polymers. The use of such release agents causes the following troubles. When the release agents are applied to the mold by a spray coating method, the releasability greatly varies depending upon the amount of the release agents applied and the number of molding shots after the spray thereof, and furthermore, the molded articles are poor in luster. Accordingly, the molded articles are not suitable for the applications where importance is set on their appearance and precision of dimension. When the release agents are applied to the mold by a coating and baking method, the baked release agent coat tends to be partially separated from the mold thereby influencing the releasability and the luster of the molded articles. Moreover, it requires substantial time and labor to restore the partially separated release agent coat, i.e., to scrape down the coat from the mold and again coat and bake the release agent on the mold.
The second difficulty encountered in the method wherein an organic peroxide-incorporated high fluidity polyethylene is used, is that the organic peroxide decomposes to produce low molecular weight radicals which take hydrogen from the polyethylene to form low boiling point substances. Such low boiling point substances cause bubble formation in the molded articles and emit an offensive odor.
In order to avoid the bubble formation, it has also been proposed in Japanese Patent Publication No. 18,296/1979 to use an ethylene polymer composition comprised of an ethylene polymer having incorporated therein an acetylenically unsaturated peroxy compound and a diolefin polymer. The crosslinking acetylenically unsaturated peroxy compound decomposes to produce low molecular weight radicals, but these radicals react on the diolefin polymer to form high molecular weight radicals which do not cause bubble formation in the molded articles. However, this proposed method still has the difficulties that the use of a release agent is unavoidable, the molded articles are poor in luster, and an offensive odor is emitted which irritates the eyes of an operator when the molded articles are demolded.
It has now been found by the inventors of the present invention that a polyethylene composition comprised of polyethylene having incorporated therein a combination of certain crosslinking agent and crosslinking aid results in rotationally molded articles which are demoldable without use of a release agent and exhibits good luster. However, if this polyethylene composition is rotationally molded at a sufficiently high temperature for providing a molded article exhibiting an enhanced degree of crosslinking and thus a satisfactory impact resistance, gases generated by the crosslinking reaction migrate to the interface between the mold surface and the fused polyethylene layer, thereby causing a plurality of depressions (hereinafter referred to as "craters") on the surface of the fused polyethylene. Furthermore, the pressure of gases present in the craters causes cracks in the areas adjacent to the craters on the surface of the crosslinked, fused polyethylene due to the fact that the crosslinked, fused polyethylene has little or no elongation. Thus, it is difficult to enhance the impact resistance without affecting the appearance.